This invention relates to an X-ray exposure apparatus for use in a lithographic process in manufacture of a semiconductor memory, for example, using exposure light such as an X-ray beam, particularly, synchrotron radiation.
X-ray exposure apparatuses generally have a structure that an X-ray source is placed in a vacuum ambience while the exposure apparatus is placed in an atmosphere or a reduced pressure ambience. At the interface between them, an X-ray window which can transmit desired X-rays but which can serve as a pressure partition wall is disposed. Such X-ray window uses a beryllium film. Since the production of a beryllium film uses a rolling process, there may occur small non-uniformness in thickness or in density of a few microns to a few millimeters. Such non-uniformness of thickness is adversely influential to the X-ray transmission factor, causing non-uniformness of intensity in the illumination X-ray beam in the X-ray exposure apparatus. Since such non-uniform intensity is directly transferred to a resist, exposure non-uniformness is produced.
Some proposals have been made against such problem. Japanese Published Patent Application, Publication No. 64451/1993 proposes a method wherein a window frame having a beryllium film adhered thereto is vibrated to cause vibration of the beryllium film, to thereby reduce the non-uniformness in the transmission factor. FIG. 1A shows a structure based on this method. In the drawing, there are an X-ray introduction tube 61 at an X-ray radiation source and another X-ray introduction tube 62 at an exposure apparatus side, and a partition wall 63 is formed between these introduction tubes. The partition wall 63 has an opening hole 63a for passing an X-ray beam 64 therethrough. Inside the X-ray introduction tube 62 of the exposure apparatus, a window frame 66 having a beryllium film 65 fixed thereto is held by a holder 67. The window frame can be vibrated in directions perpendicular to an X-ray radiation orbital plane. A vibrating mechanism therefor comprises a vibrating drive means 68 connected to the holder 67 through a transmitting means 69. The vibrating drive means 68 is disposed outside the X-ray introduction tube 62 of the exposure apparatus. Further, an edge portion of the opening hole 63a of the partition wall 63 as well as the window frame holder 67 are gas-tightly connected to each other, by means of a bellows 70, such that the vacuum ambience at the X-ray light source side is maintained.
Further, Japanese Laid-Open Patent Application, Laid-Open No. 100049/1992 shows a specific example of a window vibration width of 5 mm, a reciprocation frequency of 1 Hz, and an exposure time of 10 min.
However, where vibration is applied to an X-ray window as proposed conventionally, there occur undesirable problems as follows.
When it is assumed that the vibration is simple oscillation with an amplitude 2a and a period xcex, the vibration can be defined as follows:   y  =      a    ⁢          xe2x80x83        ⁢    cos    ⁢                  2        ⁢        π            λ        ⁢          (              t        +        θ            )      
In this vibration, the probability of existence is expressed by the following equation:       P    ⁡          (      y      )        =      1          π      ⁢                                    a            2                    -                      y            2                              
FIG. 1B illustrates the window position as represented by these equations, and FIG. 1C shows the probability of existence. As seen from these drawings, when vibration is made, there is a second whereat motion is stopped momentarily at the reverse for the reciprocal movement. In an idealistically triangular wave motion, there is no such moment included. However, it is very difficult to vibrate a particle having a mass in accordance with an idealistic triangular wave. Practically, the vibration follows a sine wave or a wave close to it. Namely, it follows a wave such as shown in FIG. 1B. If an exposure process is performed while such vibration is applied to an X-ray window, the probability of existence becomes uneven during a single exposure time period. As a result, non-uniform transmissivity information at zero-speed positions, at the opposite ends of the vibration, is transferred to a resist. Where vibration of repeated reciprocations is applied during the exposure time period, the probability of existence of the window position is largely collected at the opposite ends of vibration. Consequently, the exposure time at the opposite ends of vibration of the X-ray window is prolonged, and thus a resultant exposure illuminance distribution becomes like one to be produced when X-ray windows are held at the opposite ends of vibration and exposure amounts there are combined. For these reasons, non-uniformness of exposure can not be removed sufficiently.
In regard to the relation between the exposure time and the vibration period, if vibration is made at a period shorter than the exposure time, similar problems as described above will arise. If vibration is made at about a period corresponding to the exposure time, it means that there is at least one reverse during the exposure. Thus, non-uniform transmissivity information at that position is transferred to a resist. Particularly, in an exposure process using synchrotron radiation the intensity of the synchrotron radiation light may be attenuated with time, and in that occasion the exposure time has to be prolonged in inverse proportion to it. Namely, the ratio of the exposure time period and the non-exposure time period changes with time. In such cases, it is difficult to hold an optimum relation between the exposure time and the vibration of the X-ray extracting window.
In Japanese Laid-Open Patent Application, Laid-Open No. 100049/1992 mentioned above, the vibration width of the X-ray window is about 5 mm. However, it needs a very complicated mechanism for vibrating an X-ray window (which should function also as a pressure partition) by several millimeters or several ten millimeters. If such a large vibration is applied, vibration components will be propagated to the major assembly of the exposure apparatus, causing adverse influences to the exposure process. The structure of the exposure apparatus will be more complex since some measures have to be taken to prevent it. Further, in order to allow vibration application, the X-ray window should be fixed flexibly by means of a bellows, for example. If the amplitude is large, it causes fatigue of the bellows which may lead to breakage of vacuum.
It is an object of the present invention to provide an X-ray exposure apparatus by which non-uniformness of exposure to be produced by non-uniformness of transmission factor of an X-ray extracting window can be averaged uniformly and assuredly, such that the non-uniformness of exposure can be reduced sufficiently.
It is another object of the present invention to provide an X-ray exposure apparatus by which the structure can be simplified and by which the cost can be decreased significantly.
It is a further object of the present invention to provide an X-ray exposure apparatus by which any load applied to a bellows can be reduced, such that the apparatus fully meets mass-production.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.